A method and device for controlling a fuel metering system of an internal combustion engine is discussed, for example, in German patent document no. 199 45 618. In this method, the activation duration of at least one electrically operated valve determines the fuel quantity to be injected. The minimum activation duration during which fuel is only just injected is determined in certain operating states. The activation duration is increased or reduced, starting at an initial value. The activation duration during which a signal undergoes a change is stored as the minimum activation duration. The signal used is a quantity characterizing the uniformity of rotation, an output signal of a lambda sensor, or an output signal of an ion current sensor. This method allows for an injection quantity drift over the lifetime of the fuel injector in the range of the pilot injection.
Non-prepublished German patent document no. 102 15 610 discusses a system and method for correcting the injection behavior of injectors, in which, in order to increase the product output, an injector fuel-quantity compensation is carried out at several test points, which may be 4 test points, i.e., during the pilot injection, during idle operation, at the emission point, and during full-load operation.
This injector fuel-quantity compensation is necessary because injectors of that kind have different fuel quantity maps due to their mechanical manufacturing tolerances. A “fuel quantity map” is understood to be the relationship between the injection quantity, the rail pressure, and the activation time (activation duration). As a consequence, in spite of the electrically defined control, each individual injector fills the combustion chamber with different fuel quantities.
In fact, to achieve as low a fuel consumption as possible while complying with strict exhaust emission standards, the injectors may only have very small tolerances in terms of the injection quantity during operation. These small tolerances required cannot be met due to the mechanical manufacturing tolerances. In order to nevertheless guarantee a defined injection quantity of the injectors, the injectors are measured for their injection quantity at characteristic operating points or test points after manufacture, and are classified. The respective class must be known to the engine control unit during the operation of the internal combustion engine so that the control can be adapted to the particular characteristics of the class specifically for each injector. The class information is stored on the injector, for example, by different codes such as a bar code, by resistors on the injector, or by plaintext on the injector.
Moreover, electronic storage means, in which are stored, for example, the class information, may be provided in the injectors. These values may be read out from the injector by the control unit via an interface and used in the subsequent operation.
Over their lifetime, such common rail injectors are observed to have a fuel quantity drift, which is different for each individual injector and depends, for example, on the load profile or on the type of injector. This fuel quantity drift has a negative effect in terms of low fuel consumption, on compliance with strict exhaust emission standards, and with respect to, for example, the noise level of the internal combustion engine. Until now, correction of the injection quantity drift over the lifetime of the injectors was only done in the pilot injection plateau using a method discussed in German patent document no. 199 45 618. In contrast, fuel quantity drifts at other operating points can only be compensated for to a very limited degree, if at all.